The present invention relates to a form of indicator capable of binary indication of a fault or malfunction condition, for example, through movement of one of a pair of relatively movable indicator discs. The invention in particular relates to an electromagnetic indicator, many forms of which presently exist in the prior art.
One prior art form of electromagnetic indicator is the indicator disclosed in U.S. Pat. No. 3,704,462 which issued to George E. Pihl on Nov. 28, 1972. Reference may be had to the patent for full particulars as to its operation.
Briefly stated, however, the Pihl patent describes an electromagnetic indicator which is responsive to a condition as those above whereby one of a pair of cooperating indicator members, each of which is of disc-shape and mounted along a common axis, rotates relative to the other to change a visual display. Both of the indicator members have a plurality of sectors of like size; the sectors of the stationary indicator member being alternately transparent and opaque, while the sectors of the movable indicator member are totally opaque although alternately distinguishable. In the Pihl patent, it is described that the alternate sectors of the movable indicator are white and dark and that the opaque sectors of the stationary indicator member are dark, also. Pihl discusses that the dark sectors of both indicator members may be colored with a black paint. Thus, in one position, for example, the "set" position, of the movable indicator member its black sectors align with black sectors on the stationary indicator member and its white sectors align with transparent sectors, also on the stationary indicator member, and in the position to which the movable indicator member will have moved by virtue of a fault or malfunction condition the opposite alignment will be seen. An alignment of the sectors of the movable indicator member also may be employed such that in the "set" position its black sectors align with the transparent sectors of the stationary indicator member. For purposes of the present application, particularly in the discussion of the operation of the electromagnetic indicator, it will be assumed that in the "set" position of the indicator members the display will be totally black and that a fault or malfunction condition will be communicated displaywise by alternating black and white sectors.
Electromagnetic indicators, such as the electromagnetic indicator disclosed in the Pihl patent, while generally acceptable have been found to suffer from certain disadvantages. These relate principally to the manner of damping the return movement of the resetting structure which is actuated for returning the movable indicator member to the "set" position from which it shall have moved upon occasion of a fault or malfunction condition and the manner of proper tensioning of a spring which is provided to enable the return movement. In Pihl, resetting movement of the resetting structure is against the bias of a return spring and damping of the resetting structure is provided by a damping medium which acts to decelerate and prevent snap action return of the resetting structure driven by the return spring. The damping medium is a flowable viscous liquid, a volume of which is confined within the electromagnetic indicator in the space between the resetting structure and the stationary structure. The use of a viscous liquid has been found to create problems both of filling and front sealing of the electromagnetic indicator. And there are problems in the proper tensioning of the return spring which, at best, is accomplished through a trial and error procedure in the assembly of the electromagnetic indicator. Thus, the manner of tensioning of the return spring is by means of receipt of one end thereof in a slot carried by a component of the resetting structure of the electromagnetic indicator and by receipt of the other end in one of a plurality of slots carried by a component of the stationary structure of the electromagnetic indicator, thereby being dependent upon which of the slots receives the other end of the spring.